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An acrylic painting by Don Cooper and Leah Leverich shows the transition zone between densely packed pyramidal neurons in the CA1 region.

In spring, days grow long, and the white-footed mouse looks for a mate. For some mammals, day length prompts behaviors like breeding or camouflaging, and scientists say it’s not just the arc of the sun that kicks off these seasonal events; substances in the brain also play a part.

One important element is melatonin, a hormone that the mammalian brain secretes at night. According to a study conducted at Ohio State University, changes in the duration of melatonin secretion not only affect the behavior of white-footed mice, they also induce changes in their brains.

The group’s previous research showed that white-footed mice held in short winter-like days showed changes in the mechanism behind how memories are formed and stored in the brain, which they say is associated with impaired spatial learning and memory.

To test the hypothesis that melatonin plays a role in brain plasticity in the hippocampus, a brain region associated with these functions, the research team studied male white-footed mice. They implanted empty or melatonin-filled capsules into the rodents, then exposed the animals to long or short day lengths for ten weeks.

After testing the animals’ behavior, the scientists confirmed that mice exposed to short day lengths, or increased melatonin, displayed impaired spatial memory. They then used Neurolucida to analyze cell morphology in Golgi-stained brain sections, discovering that different types of hippocampal neurons reacted differently to extended melatonin exposure. CA1 pyramidal neurons showed shorter dendrites and reduced “dendritic complexity,” while the opposite was true for CA3 neurons. Spine density also decreased in CA1 neurons, and soma size was reduced in dentate gyrus granule cells.

“The current results demonstrate that chronic melatonin implants reproduce the effects of short days on the hippocampus and implicate melatonin signaling as a critical factor in day-length-induced changes in the structure and function of the hippocampus in a photoperiodic rodent,” the authors say in their paper.

These changes may be important on an ecological level the authors add, pointing out that during the long days of spring and summer, male white-footed mice maintain and defend their breeding territories – behaviors that require “enhanced hippocampal function and spatial memory.” But in the winter, the weakening of hippocampal functions may be a way for the animals to “conserve energy to survive the harsh energy-restricted short days of winter.”